4.3. Understanding pH: Acids, Bases, and Indicators

The Hydrogen Index (pH)

• Litmus can differentiate whether an environment is acidic or basic.
• A universal indicator is used to determine how acidic or basic an aqueous solution is.
• This indicator changes color based on the degree of acidity or basicity of the environment.
• Not all acids have the same acidic properties and the same is true for bases. Some bases and acids are weak, while others are strong.
• Acids and bases can be ranked according to their strength, either increasing or decreasing.
• The strength of acids or bases is determined by the pH value, otherwise known as the hydrogen index and takes values from 0-14.
• The color of the universal indicator shows the pH in a substance.

Types of Substances

• Strong acid: HCl (hydrochloric acid) turns red
• Weak acid: H2CO3 (carbonic acid) turns green to yellow
• Neutral: H2O (water) turns green
• Weak base: NH4OH (ammonium hydroxide) turns blue
• Strong base: NaOH (sodium hydroxide) turns violet
• pH values from 0-6 indicate an acidic environment.
• pH values from 8-14 indicate a basic environment.
• A pH value of 7 indicates a neutral environment.

Activity

• Prepare aqueous solutions of different substances (acids, bases, salts). Use the universal indicator for each solution, noting any color change. Record the colors given by the universal indicator in each solution. Compare these with the colors on the pH scale to determine the pH value. Rank the solutions tested according to their properties: weak acid, strong acid, neutral, strong base, or weak base.

Activity Results

• Lemon juice has a universal indicator color of yellow, a pH value of 4 and a weak acid reaction to the environment
• NaOH solution has a universal indicator color of blue, a pH value of 8 and a weak base reaction to the environment
• Soap solution has a universal indicator color of green, a pH value of 8 and a weak base reaction to the environment
• Coca-cola has a universal indicator color of yellow, a pH value of 4 and a weak acid reaction to the environment

Summary

• The pH value indicates how acidic or basic an aqueous solution of a substance is.
• The universal indicator changes color each time the pH value of a solution changes.
• For pH values less than 7, the environment is acidic.
• For pH values greater than 7, the environment is basic.
• If the pH value is 7, the environment is neutral.

Explore the pH scale and how it measures the strength of acids and bases. Learn how universal indicators change color to reflect pH levels. Discover examples of strong and weak acids and bases, and their corresponding pH values.